With a recent tendency of rapid decrease in weight of portable electronic equipment etc., it is required to minimize a weight and size of the alkaline battery to serve as a power supply for the equipment. That is, the alkaline battery is required to be increased in its energy density.
Various types such as a sintered type, a pasted type, a button type and a pocket type etc. have been known as the nickel plate for use as a positive electrode of the alkaline battery.
The sintered type nickel plate is one, in which powder of positive active material having a principal component of nickel hydroxide is filled in a porous sintered substrate formed by sintering nickel powder to a porous substrate such as a punched steel plate or a nickel net etc. In this electrode plate, peeling-off of nickel sintered product will occur due to weakening of mutual bonding between sintered nickel powder particles when a porosity of the substrate is increased. Therefore, a limit of practical porosity has been about 80%. For this reason, a filling density of active material has been small and only an energy density as small as about 400 mAh/cc has been obtained. Since pore diameters of the substrate have been as small as 10 microns or smaller, a method for filling the positive active material has been limited to a solution impregnation method requiring repeated troublesome processes.
The button type nickel plate is one, in which a small quantity of conductive material such as carbon powder is added to the foregoing positive active material powder, and this composite powder is pressed to be formed into a pellet shape. The pocket type nickel plate is one, in which a punched steel plate is worked to form a pocket and the foregoing positive active material powder is filled in this pocket. In these electrode plates, the positive active material powder is filled directly and the substrate to which the nickel powder is sintered is not used. Consequently, these plates have lacked in a current collecting property and offered an active material utilization efficiency as small as about 50% to 60%, so as to exert a performance considerably inferior to that of the sintered type nickel plate offering an active material utilization efficiency as large as about 90%. In order to improve these disadvantages, a measure in which fine particles such as nickel carbonyl powder are mixed to the foregoing positive active material powder is taken at present. Even by this measure, however, the performance can be improved by only about 10 %. The reason why the mixed nickel carbonyl powder does not function effectively is that nickel hydroxide having a poor electric conductivity is formed on surfaces of the nickel carbonyl powder particles at a positive electrode charge/discharge potential.
The pasted type nickel plate is formed in such a way that 1 to 30 wt % of CoO (cobalt monoxide) powder are mixed to the foregoing positive active material powder, this mixture is formed into a paste by using solvent such as MC (methyl cellulose) and CMC (carboxymethyl cellulose) etc., and this paste is filled in a nickel fiber porous plate. This electrode plate is one, in which CoO is dissolved in electrolyte solution and made precipitate around the porous plate and the positive active material as .beta.-Co(OH).sub.2 which is formed by charging thereafter to form a conductive network comprising .beta.-CoOOH having a good electric conductivity, and a discharge performance is thereby made favorable. In this electrode plate, such an aging process has been required in advance of the above charging that the plate has been put in a still condition for one to three days after filling the electrolyte solution. If the plate is not subjected to the aging process, a dissolution reaction of CoO shown in a formula (I) and a precipitation reaction of .beta.-Co(OH).sub.2 shown in a formula (II) will scarcely occur, and formation reactions of conductive network shown in formulas (III) and (IV) will become difficult so as to lower an active material utilization efficiency. EQU CoO+OH.sup.- .fwdarw.HCoOC.sup.- (I) EQU HCoOO.sup.- +H.sub.2 O.fwdarw..beta.-Co(OH).sub.2 +OH.sup.-(II) EQU HCoOO.sup.- .fwdarw.CoOOH+e.sup.- (III) EQU .beta.-Co(OH).sub.2 .fwdarw..beta.-CoOOH+H.sub.2 O+e.sup.- (IV)
However, the pasted type nickel plate has included the following problems.
(1) Since a solubility of cobalt in the electrolyte solution is low and a quantity of the electrolyte solution is small, so that a reaction of the formula (I) does not take place sufficiently. For this reason, in order to form the conductive network sufficiently, it has been necessary to carry out the aging process for a long period or to repeat charging and discharging of several cycles, so that a productivity has been poor. PA1 (2) Dissolved cobalt complex ions disperse to precipitate on various places as .beta.-Co(OH).sub.2, so that a short-circuiting is apt to occur when they precipitate on a separator, and hydrogen gas is produced violently to lower a hydrogen overvoltage so as to increase a self-discharge at a negative electrode when they precipitate on the negative electrode, if the negative electrode is a zinc electrode. Therefore, in case when applying the nickel plate to the nickel-zinc battery, it has been necessary to assemble the nickel plate into the battery after the plate had been previously formed separately from the zinc electrode to convert CoO into .beta.-CoOOH. For this reason, many manufacturing processes have been required and an equipment for charging has been necessary. In addition, peeling-off of the active material and breakage of the conductive network have taken place because the plate has been subjected to processes such as formation and rinsing etc., so that there have been such problems as a decrease in quantity of the active material and a lowering of the active material utilization efficiency etc. PA1 (3) CoO is oxidized to become Co.sub.3 O.sub.4 and is not converted to .beta.-CoOOH when it is left as it is in air. Therefore, a battery performance based on CoO can not be obtained stably.